The present invention relates to a torque limiting device. More particularly, the invention relates to a restraining mechanism which provides a relatively constant torque through a predetermined range of motion.
Torque limiting devices support objects at selected angular positions relative to a main body. An example of such a torque limiting device is a spring hinge which utilizes a helical spring wrapped around a shaft. When the shaft is rotated, one of the spring ends or toes contacts a stop or stationary support to cause the spring to slip relative to the shaft. Such a spring hinge can be designed to provide fairly constant torque or resistance to rotation throughout its range of motion. In order to provide a constant torque in these spring hinges, a wrap open is often introduced into the operation of the spring hinge. The wrap open occurs as the shaft is initially rotated. The spring toes engage the stationary support or stop to cause the spring to expand or open, i.e., "wrap open". This loosens the interference grip of the spring on the shaft and allows the shaft to be rotated relative to the spring upon application of a specified torque or force.
One of the problems with spring hinges is that a spring back is introduced into the operation of the spring hinge. Spring back is caused by the spring wrapping open during movement caused by an external torque applied to the shaft, and then wrapping back down on the shaft once the torque is removed. Spring back is also caused by the toe supporting the load. The toe will deflect as it supports the load during the movement and then relax after the load is removed. Both of these conditions cause the hinge to move the load in the opposite direction of the applied torque after the load is removed.
More particularly, when the applied torque is removed from the shaft, the spring relaxes to a more closed condition, i.e. it "wraps down" onto the shaft, causing the toes to apply a force on the stops, thereby rotating the shaft oppositely to the direction of rotation causing the wrap open. Similarly, when the applied force is removed, the toes relax from their deflected state, also rotating the shaft oppositely to the direction of rotation causing the deflection.
Prior art spring hinges also require very accurate machine tolerances. The spring toes of the spring winding must terminate precisely at the stationary support structure or stop. Inaccuracies in the spring ends will lead to additional spring back of the spring hinge. In addition, the precise location of the spring toes relative to the stop is critical to the performance of the prior art spring hinges. If the angle between the spring toe and the stop is too open, the hinge will have free play. That is, there will be no torque for a limited range until the spring toe engages the stop. In addition, if the angle between the spring toe and the stop is too closed, the toes will make the spring hover over the shaft resulting in no or low torque.
In addition, the longevity of spring hinges is compromised when grease which is typically present in the spring escapes from the hinge thereby allowing the spring hinge to wear out. Similarly, spring hinges are susceptible to rusting as most spring hinges are not enclosed and are exposed to the elements. Rusting will negatively impact the performance of the spring hinge.
The present invention solves these and other problems associated with the prior art.